The present invention relates to a shadow mask type color picture tube and, more particularly, to the structure of a shadow mask assembly.
In general, a color picture tube of the shadow mask type has a tube envelope which is constituted by a panel section, a funnel section and a neck section. Three electron guns are arranged in the neck section. A color selecting electrode assembly, i.e., the shadow mask assembly, is disposed in the panel section adjacent to a phosphor screen formed on an inner surface of the panel section of the tube envelope. The shadow mask assembly is constituted by a rectangular shadow mask having a color selecting function with respect to three electron beams emitted from the electron guns, and a frame for mounting the shadow mask on the panel section at a given distance from its inner surface.
The shadow mask has a main surface and a skirt portion extending from the main surface along the tube axis. The main surface has a central apertured portion with apertures each for allowing electron beams to pass through, and a nonapertured peripheral portion extending toward a peripheral portion of the main surface. The frame is disposed inside the skirt portion. The skirt portion of the shadow mask is fixed to an outer periphery surface of the frame by spot welding at a plurality of locations. The frame has a holder for fixing the shadow mask to the panel section at the given distance between the shadow mask and the phosphor screen. Low cost iron is used for the shadow mask and frame.
In the color picture tube of shadow mask type, three electron beams emitted from the electron guns are electromagnetically deflected,and thereafter, impinge to predetermined phosphors, e.g., R, G and B phosphors, through an aperture of the shadow mask serving as the color selecting electrode. However, when the color picture tube is operated, the shadow mask itself is thermally expanded by the incident electron beams. For this reason, the relative positional relationship between the center apertured portion of the shadow mask and the phosphor screen is changed. Thus, the location at which the electron beam impinges to the phosphor screen varies, and in an extreme case, the color purity of the phosphor screen is considerably degraded.
Generally, deformation of the shadow mask due to thermal expansion when the color picture tube is operated can be classified into two types.
In the first type, the temperature of the whole shadow mask is increased. Thus, the entire shadow mask and the frame are thermally expanded in a direction perpendicular to the tube axis. In the second type, if a particularly bright portion is locally present on the screen when the color picture tube is operated, a portion of the shadow mask corresponding to the bright portion is locally heated. In general, the portion of the shadow mask then thermally expands so as to extend along the tube axis in a so-called dome shape.
In the first type of deformation of the shadow mask, as disclosed in Japanese Patent Publication No. 44-3547, a bimetal member is interposed between the frame and the holder. The shadow mask assembly can be automatically compensated for by means of the bimetal member so as to approach to the phosphor screen formed on the inner surface of the panel portion in accordance with the increase in temperature of the entire shadow mask.
Meanwhile, as for the second type of deformation, since the thermal expansion occurs locally when the color picture tube is operated, no effective compensation method has yet been found as in the bimetal member mentioned in the first type of deformation, but various proposals have been made.
In one proposal, as disclosed in Japanese Patent Publication No. 42-25446, Japanese Patent Laid open Nos. 50-58977 and 50-68650, a material having a low thermal expansion coefficient, e.g., an invar alloy is used for the shadow mask assembly. However, such a metal having a low thermal expansion coefficient, e.g., invar alloy is expensive compared to iron. Therefore, the use of such a metal not only for the shadow mask but also for the frame results in a considerable increase in cost. Therefore, it is preferable that the metal having low thermal expansion coefficient be used only for the shadow mask which is relatively light in weight, and a low-priced iron material be used for the frame which is relatively heavy, because the volume of the frame is much larger than that of the shadow mask.
However, if different metals are used for the shadow mask and the frame, when the temperature of the overall shadow mask assembly becomes high, e.g., when the color picture tube is manufactured, the shadow mask is deformed due to the difference in thermal expansion coefficients. That is, as shown in FIG. 7, when the temperature of an overall shadow mask assembly 10 becomes high, a frame 12 made of an iron material having a high thermal expansion coefficient is greatly expanded, so that the outer periphery surface of the frame 12 pushes the skirt portion of a shadow mask 14 through a plurality of welding portions 15 between the shadow mask 14 and the frame 12, as indicated by arrows 16. On the other hand, since the shadow mask 14 is made of a material having a low thermal expansion coefficient, the shadow mask 14 itself is deformed very little. Thus, as indicated by imaginary lines 17, the shadow mask 14 is plastically deformed by tension from the entire outer periphery surface of the frame at its surrounding portion, i.e., at the periphery of its nonapertured and apertured portions along the tube axis in a direction away from the phosphor screen. Plastic deformation at the apertured portion degrades color purity on the phosphor screen.
In the above proposal, since the frame is formed smaller in size than a space defined by the inner side surfaces of the skirt portion of the shadow mask, a portion of the frame is disposed inside the skirt portion of the shadow mask. Finally in U.S. Pat. No. 4,056,755 (Sohn), the frame is formed larger than a space defined by the outer side surfaces of the skirt portion of the shadow mask, and a portion of the frame surrounds the skirt portion. However, in this case, the shadow mask and the frame are made of the same material, and no solution for plastic deformation of the shadow mask when different materials are used is disclosed.